Small urban transport vehicles are convenient for the limited amount of parking space they require. They also require less energy to move given their reduced mass. Small off-road vehicles are used on rougher terrain and offer similar advantages. Such vehicles typically have recumbent seats or a single straddle seat, like all-terrain vehicles (also known as “ATVs”).
These light mass vehicles have three or four wheels. In the case of a three-wheeled vehicle, two different configurations are generally known. The first configuration has two wheels at the front and one wheel at the back. The second configuration has one wheel at the front and two wheels at the back.
The height of the center of gravity (CG) of a vehicle has a significant influence on the dynamic stability of the vehicle. The vertical position of the CG is measured as a distance from the ground when the vehicle is at rest. For a vehicle having a straddle seat, the elevated position of the seat generally results in a high CG. This is a factor that particularly affects the stability of a light mass vehicle using a straddle type seat. The position of the center of gravity also changes according to the presence and the driving position of the driver on the straddle seat. The presence of a passenger also has a significant effect given that the additional mass of the passenger accounts for a significant portion of the mass of the loaded vehicle.
Recumbent type seat vehicles are generally more stable since they have a lower CG when loaded but they require additional space. Recumbent type seats include bucket seats of the type usually found in four-wheeled vehicles. Recumbent seat configurations in a four-wheeled vehicle generally position two riders side-by-side.
While straddle seats may alter disadvantageously the center of gravity of a vehicle, they offer certain advantages that are not available with recumbent seats. In particular, straddle seats allow the driver to adopt a more compact riding position, allow for a better vision since the driver is disposed higher, and permit the rider to lean into a turn for enhanced handling.
An advantage of a vehicle having a tandem straddle type seat, which can accommodate a driver and a passenger behind the driver is that the center of gravity of the vehicle remains laterally symmetrically positioned when the vehicle is upright regardless of whether a passenger is present or not. In contrast, on a light mass recumbent vehicle having side-by-side seats, when only the driver is present, the center of gravity is not laterally located in the same position as when there are two riders in the vehicle. When only a driver is present onboard a vehicle with side-by-side recumbent seats, the center of gravity will be offset from the longitudinal centerline of the vehicle in a direction toward the driver. As would be appreciated by those skilled in the art, this offset may have an effect on the handling performance of the side-by-side recumbent seat vehicle.
Other factors that affect stability include the distance between the tires —the track width. On a vehicle, the wheel base refers to the distance between the front tire(s) and the rear tire(s). The wheel track, on the other hand, refers to the distance between two tires on the same axle. A larger distance between the tires (whether it be the wheel base or the wheel track) enhances the stability of the vehicle, but creates a larger vehicle, in terms of overall length and width, that may be less manoeuvrable because of the vehicle's increased size.
When operating any vehicle, especially a three-wheeled vehicle, stability is a concern during turning. When negotiating a curve, a vehicle is subject to centrifugal forces, as is readily understood by those of ordinary skill in the art of vehicle design. Generally, a higher center of gravity causes the vehicle to have a lower rollover threshold than a vehicle with a lower center of gravity due to centrifugal forces.
Three-wheeled vehicles raise special stability concerns since they have a smaller total footprint area with the ground than a similar sized four-wheeled vehicle. Also, three-wheeled vehicles tend to have a smaller mass. Therefore, they are also more affected by load variations, such as driver, passenger and cargo mass.
As would be appreciated by those skilled in the art, modern road tires can offer considerable grip on a road surface. The gripping force of modern road tires can be so strong, in fact, that a vehicle with a high center of gravity may be subjected to centrifugal forces that may cause the vehicle to exceed its rollover threshold. If the rollover threshold is exceeded, one or more of the vehicle's wheels on the inner side of the curve may lift off of the road surface which may lead in some circumstances to the vehicle rolling over. Rollover can also occur under severe over steering conditions when the tires suddenly recover traction with the ground or hit an obstacle sideways.
For these reasons, Electronic Stability Systems (ESS) have been developed to improve the stability of such vehicles.
ESS, also known as Vehicle Stability Systems (VSS), are designed to electronically manage different systems on a vehicle to influence and control the vehicle's behaviour. An ESS can manage a considerable number of parameters at the same time. This provides an advantage over a vehicle having no such system since the driver can only manage a limited number of parameters at the same time and has a slower reaction time. A typical ESS takes several inputs from the vehicle and applies different corrective measures back to the vehicle to influence the vehicle's behaviour. Examples of inputs include steering column rotation, longitudinal and transverse acceleration of the vehicle, engine output, brake and accelerator pedal displacement, rotational speed of the wheels, and brake pressure in the brake system amongst others.
The outputs from the ESS affect the vehicle's behaviour generally by independently managing the brakes on each wheel, the suspension, and the power output of the engine in order to improve the vehicle's handling under certain circumstances.
However, since the load (rider, passenger, cargo mass) applied to light mass vehicle has a significant impact on its handling characteristics, as previously mentioned, the ESS may take insufficient corrective measures when the vehicle is heavily loaded or may unnecessarily limit the vehicle performance when the vehicle is lightly loaded depending on the ESS calibration.
Therefore, there is a need for a system that controls the stability of a light mass vehicle that takes into consideration the overall mass of the vehicle (vehicle, driver, passenger).